Help with grafted cactus... Sick or dying..!!? :)?

I have a small grafted cactus... And on the top pink part, part of it is turning brownish, and caving in/shriveling up a bit. It's not as bright of a color as it used to be either. I put it in some new dirt, and it normally doesn't have sunlight because we don't gave much... Does a natural light lamp work for that?? So if anybody knows what's wrong with it, and can tell me what to do if I can save it it'd be awesome!!! Also, the base of it seems fine... Just the top is getting weird...:)